1. Field of the Invention
This invention relates to hydraulic working oil compositions for use in buffers and more particularly to such oil compositions suitable for use in car suspension devices such as shock absorbers, active suspensions, stay dampers and engine dampers.
2. Prior Art
As conventional hydraulic working oils which have hitherto been used in car buffer devices such as shock absorbers, active suspensions, stay dampers and engine dampers, there have been known those incorporated with a phosphoric acid ester and/or a phosphorus acid ester to provide the car buffer devices with friction-reducing properties and wear-preventing properties. In addition, there have also widely been used such hydraulic working oils in which are additionally used oily agents such as a fatty acid, aliphatic alcohol and fatty acid ester to further improve the working oils in friction-reducing properties.
Hydraulic working oils are those which are required to be capable of reducing friction at friction surfaces simultaneously with preventing wear of the friction surfaces. Recently, there have been increasingly used bush members impregnated with a Teflon resin in attempts to reduce friction at friction surfaces by having resort to such material or substance as above. Further, gas-sealed type and damping force-variable type buffers have particularly been increasingly used and, therefore, load applied to the friction surfaces of the buffers has been increased whereby conditions under which the buffers are used have come to be severe.
Consequently, Japanese Patent Application Laid-Open Gazette No. Hei 5-255683 (No. 255683/93) discloses, as a hydraulic working oil exhibiting excellent wear resistance and Friction characteristics even under severe conditions, a composition comprising a base oil which contains therein a phosphorus-containing compound such as a phosphoric acid ester or phosphorous acid ester and a nitrogen-containing compound comprising C.sub.12 -diethanolamine. The present inventors also found out that compositions comprising as essential components a phosphorus-containing compound such as a phosphoric acid ester or phosphorous acid ester, and a nitrogen-containing compound having a specific structure, in addition to a base lubricating oil, are particularly excellent in durability (little degradation with the time of use) of friction-reducing effect as a hydraulic working oil for a buffer, and previously filed a patent application based on this finding (Japanese Patent Application No. Hei 6-37528).
Although hydraulic working oil compositions for a buffer which have an excellent wear-preventing effect can be obtained by the combined use of the nitrogen-containing compound, which is described in the above two Japanese patent applications, and the phosphorus-containing compound; however, they have been found to raise problems because their storage stability is deteriorated so as to produce sludges when the content of the nitrogen-containing compound is increased, while their durability of friction-reducing effect is deteriorated when the content thereof is decreased to such an extent as not to worsen their storage stability.